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United States Department of Influence of Forest Structure on Service November 2003 Behavior and the Severity of Its Effects

An Overview

and 2002: , an abundance of (combustible forest materials), lack of moisture, and terrain characteristics. Of these four factors, only fuel abundance can be directly influenced through human intervention; however, the treatments to reduce can significantly modify behavior and severity and reduce environmental damage caused by fire. Excluding fire from the natural cycle has resulted in a buildup of flammable materials across large areas of the forest . As forest fuels accumulate, the forest structure changes, leading to greater continuity of fuels between the ground surface and the upper canopies. This altered structure provides “ladders” for wildfire to climb up into the tree tops. Where are densely packed, the fire can spread from tree to tree in a phenomenon known as crown fire or crowning. Crown are intense and fast- moving. They can threaten communities and damage key , including timber, fish and , , and drinking quality. Crown fires are virtually unstoppable, and often result in large burned areas despite costly and dangerous fire suppression efforts. Wildland fire of the Monument Fire, OR, 2002. The effects of decreased fire occurrence on fuels and fire behavior vary from site to site. In the Introduction Western , the effects are most pronounced in arid to semiarid dominated The year 2002 was the second largest fire by ponderosa , Douglas-, or a combination in almost 50 years. Four States had the largest of the two. Historically, these areas burned in wildfire since at least the early experienced frequent fires, which typically burned 1900s—Arizona, , New Mexico, and at low intensity in surface fuels, with only . wasn’t far behind. Across the occasional damage to overstory trees. Removing Nation, almost as many acres burned in 2002 as in 2000, when the area burned was twice the 10-year average. The 2002 season could have been much worse. The Forest Service and its partners in the community controlled 99 percent of the wildland fires during initial attack. The fires that did escape initial attack threatened and property, compromised municipal watersheds, and damaged ecosystems—many of which will take decades to recover. Fire suppression expenditures for 2002 were $1.4 billion. More than 90 years of fire research shows that Grazing reduces surface fuels, which, in turn, decreases four factors working in concert can result in the the incidence of wildland fire. type of catastrophic witnessed in 2000 1 and decrease the likelihood of tree mortality from stem scorching and root damage. Surface fuel reduction also diminishes a fire’s , a key factor in the initiation of crown fires. designed to reduce crown density helps prevent tree crowns from severe, often fatal, . Reducing crown density also decreases the likelihood of a sustained crown fire moving through the forest, even if crown ignitions do occur. Proactive, -based management can improve and maintain forest health while reducing the danger of catastrophic fire in these arid and semi-arid western forests. However, the forest health and fire problems arising from fire exclusion took decades to develop and the solution will not happen overnight. The remainder of this overview presents highlights of available scientific information and implications for management action. fire from the natural cycle has resulted in overcrowded stands with dense canopies and many Influence of Forest Structure small trees of -tolerant and less fire-resistant on Wildland Fire Behavior and Effects species growing between a few large trees. This type of overall forest structure increases the A may consist of several layers of probability that a low-burning surface fire will live and dead in the , develop into a high-intensity crown fire. The midstory, and overstory—or surface, ladder, and overly dense forest structure also means that trees crown fuels. are in stressful and more susceptible Surface fuels consist of grasses, , , to disease, , and . and woody material lying on the ground. Surface Wildland fires differ greatly in severity, which fires low vegetation, woody debris, and litter. refers to the degree of impact on soils, vegetation, Under the right conditions, surface fires reduce the and other ecosystem components. Weather, forest likelihood that future wildfires will grow into structure, fuel condition, and vegetation crown fires. composition interact with location to determine a Ladder fuels consist of live and dead small fire’s severity. Land managers can design fuel trees and shrubs, live and dead lower branches treatments to alter forest structure and influence from larger trees, needles, , , , fire behavior and burn severity. Treatments that and any other combustible located reduce surface fuels tend to reduce damage to soils

Historical Present

Hypothetical simulation of changes in vertical arrangement and horizontal continuity in forest stand structure. Today’s forests are more spatially uniform, with higher densities of fire-intolerant species and suppressed trees. 2 between the top of the surface fuels and the bottom of the overstory tree crowns. Crown fuels are suspended above the ground in treetops or other vegetation and consist mostly of live and dead fine material. When historically low- density forests become overcrowded, tree crowns may merge and form a closed . Tree canopies are the primary fuel layer in a forest crown fire. Fire behavior is strongly influenced by surface, ladder, and crown fuels. Heavy ladder and crown fuels enable fires to climb upward into the crowns Hayman Fire, June 15, 2002, about 9 p.m. Photo by and help to sustain crown fires once they are Steven Smith, Colorado Department. started. Rapidly moving crown fires typically consume nearly all the fine fuels in a forest canopy easily suppressed surface fire when the when and a sloping are thrown Hayman Fire burned into the area. into the mix. Crown fires caused by excessive fuel ➢ On the other hand, all trees were killed in the accumulation are generally a severe threat to Sheepnose Fuels Reduction Project. ecological and human values and to infrastructure Although removing smaller trees dramatically and are a major challenge for fire management. reduced stand density, large amounts of Such fires kill large numbers of trees, damage surface fuels allowed the fire to burn soils, increase erosion, impair air quality, and can intensely through the stand. degrade or destroy species habitat. opens stands to greater solar and wind movement, resulting in warmer Fuel Treatments Can Influence Fire temperatures and drier fuels throughout the fire Behavior and Reduce Fire Severity season. While this openness can encourage a surface fire to spread, such fires do little ecological Where forests are overcrowded, the most damage. Where human values are threatened, these effective strategy for reducing fire risks is to design types of low-intensity fires are relatively easy to treatments to decrease fuels in all three strata. control and less likely to support a crown fire even Thinning reduces the flammability in the midstory under conditions. and overstory, while treating surface fuels, Mechanical thinning, especially when directed at including those resulting from thinning, decreases the smaller and medium-sized trees, can be quite surface fire potential. effective in reducing the conditions conducive to Three recent examples from the Hayman Fire in crown fire spread. However, thinning is most Colorado illustrate the relation between surface, effective when followed by additional treatments crown and ladder fuels, and fire behavior. such as prescribed fire to reduce surface fuels. If ➢ The Polhemus prescribed burn in November thinning is used, it is important to reduce the fine 2001 removed most surface fuel and pruned fuels, such as small branches, leaves and needles, lower live branches from trees in a ponderosa or they will add to surface fire potential. Branches, pine forest, while maintaining a desirable limbs, and other slash may need to be treated with overstory density. These changes were prescribed fire. sufficient to stop the Hayman Fire when it Piling and burning the slash and other material burned into the area in June 2002. is an effective way to treat it. If burning is not ➢ On the Manitou Experimental Forest, acceptable due to production, the residual mechanical thinning reduced density in a fuel can be removed from the site. pure pine forest and concentrated Prescribed fire is one of the most effective slash in large piles within the Trout Creek means to remove surface fuels and reduce Timber Sale. These actions resulted in an understory density. Prescribed burning affects 3 Site before thinning. During thinning operation. implementation has been guided by hundreds of scientific investigations and years of professional experience. When properly conducted with safeguards developed from experience, past research, and emerging scientific findings, these practices can be combined in a treatment regime to improve forest health, reduce fire risk, and minimize side effects on environmental conditions and ecological processes. Using them together in an integrated fashion allows them to complement one another – as when slash from thinning is Site after thinning. reduced by follow up burning, or when mechanical treatment of surface and ladder fuels reduces the potential fire behavior by reducing surface fuel probability that a prescribed fire will have loads and horizontal fuel continuity. This excessively severe impacts or undesired results. treatment limits the spread of future surface fire, decreases fire intensity, and reduces the occurrence Importance of Managing Fuels Across of spot fires. The effectiveness and appropriate- Large ness of prescribed fire varies depending on weather, initial fuel conditions, and the pattern of Removing biomass and live fuels only in the burning. Mechanical thinning followed by multiple wildland-urban interface can help protect homes applications may be required to reduce fuels gradually when ladder fuels are present. Use of prescribed fire requires caution where high surface fuel loads could lead to temperatures that might damage tree roots. It’s important to note that, with the current forest conditions in the , one treatment may not be sufficient to achieve desired conditions. Once desired conditions have been attained, maintenance treatments, such as prescribed fire, will be required to prevent excess fuel accumulation as regenerate and regrow following treatment. Thinning and prescribed burning are standard vegetation management practices whose Prescribed burn. 4 terrain, existing vegetation and fuel distribution, and probable fire paths creates landscape fuel patterns that can be expected to slow fire growth and modify behavior while minimizing the amount of treated area required. The arrangement of fuel treatment units changes fire behavior by forcing a fire to repeatedly flank around patches of treated fuels, or to burn through them at lower intensity, thus reducing the fire’s overall intensity and growth rate. These model results suggest that random fuel treatments are not efficient in Wildland-urban interface—Llamas grazing in a watch changing fire behavior. Strategic treatment is most as a wildfire comes close to home on the Deer Creek effective when areas are not impeded by outside of Selma, OR. constraints such as land ownership, endangered species, and riparian buffers. and businesses. However, protecting Another approach to managing fire on a values such as , forest health and landscape scale is the use of fuel breaks in productivity, wildlife habitat, and recreation values strategic locations, such as along ridgelines or requires vegetation and fuel management at a around communities. The concept behind fuel landscape scale. Fuel treatments carried out over breaks is to provide a defensible location for use large landscapes can reduce both the size and by firefighting crews, or to reduce fire intensity in severity of wildfires and their effects on the immediate wildland-urban interface. If the communities and the environment. Treating just a fuel break fails to stop a fire, it is unlikely to few stands in large contiguous forested landscapes influence the fire’s ultimate size and effects. will do little to modify fire behavior over the entire However, fuel breaks can be efficient and cost- area. Protecting large landscapes requires land effective in protecting homes and other structures managers to develop large-scale fuel treatment from catastrophic wildfire. As with other forms of patterns that reduce the potential for catastrophic fire and promote healthy forest conditions. Evidence from natural fire patterns that have fragmented fuels across landscapes suggests that mosaic patterns can limit the growth of large fires. This type of spatial pattern limited fire growth and altered fire behavior in such places as , National Park, and Baja California. Studies that describe patterns of fire severity after large fires and intensive studies of fire history that describe fire patterns over centuries also show that topography plays a major role in determining the patterns of fire. The influence of topography on fire behavior escalates as the slope steepens and increases in complexity. Model simulations of landscape fire behavior indicate that the spatial arrangement of treatments greatly influences the amount of area that needs to be treated. Strategic placement of treatments that A bulldozer operator knocks down trees to create a safety takes into consideration information such as zone for and fire equipment south of Show Low, AZ. Photo by Tom Schafer.

51 treatment, fuel breaks require maintenance. Fuel breaks are not a replacement for a strategic fuels treatment program. They are most effective when incorporated into a strategic fuel treatment program or community protection and preparedness program. While the long-term effects of strategic fuel treatments at the landscape level on potential fire behavior are not fully known, research indicates that the spatial mixture of stand conditions is more important than exact conditions in a given stand. This effect is evident in the mosaic patterns Fire Behavior Analyst Sue Husari from the Pacific South- of free burning fires at Yosemite National Park west Region uses a fire behavior computer model devel- and Sequoia National Park. Previous burns do oped at the Fire Sciences Laboratory to predict fire little to prevent reburning after about a decade, behavior on fire in Flagler County Florida in 1998. although they do lessen fire severity. Recently treated areas, on the other hand, can completely Model-based tools can also be used for inhibit fire spread. Therefore, a combination of scheduling fuel treatments over or for new treatments and maintenance treatments visualizing and evaluating the potential effects of arrayed across a landscape can effectively disrupt alternative treatments. For example, the fuel and fire growth and change fire behavior at the fire effects extension of the Forest Vegetation landscape scale, even though some stands within Simulator can be used to predict vegetation the landscape have not been treated recently. growth and fuel accumulation. The model also Overall, how long treatments will last and how helps managers evaluate different treatment types often they will need to be maintained will vary and schedules and their effects on fuels and future with forest type, , soils, and other factors fire behavior. Managers can evaluate fuel- that influence forest health. treatment scenarios and their effectiveness at keeping fire behavior within acceptable levels and Fire Prediction Models Can Be balance this goal with effects of treatments. In Effective as a Tool short, models and model-based decision analysis in Active tools help land managers make more informed tradeoffs. Most scientific investigation related to assessing For example, models can help managers develop the influence of fuel treatments on fire behavior at a treatment schedule to reduce the occurrence of the stand and landscape levels relies on computer crown fires in a specific landscape. The treatment models to predict outcomes. There are a variety of might include an initial heavy thinning followed by models available to predict surface and crown fire prescribed burning to reduce uncharacteristically behavior, environmental effects of fire, and the high fuel accumulations and the likelihood of fire influence of treatments on stand development. crowning. Afterward, periodic prescribed burning These models are revised and improved constantly. — every 5 to 20 years — might be sufficient to Models help to simplify much of what really control tree and surface fuels. If happens to further our understanding of the main such a schedule is not desirable or practical, factors involved in the system under study. thinning could be scheduled every 20 to 40 years, Combining these models with map-based data and perhaps accompanied by prescribed fire, to reduce analysis tools can help integrate basic landscape ladder fuels. Treatment prescriptions and analysis with fuel-treatment prescriptions for schedules may be influenced by factors such as specific stands. stand structure, species composition, site productivity, elevation, aspect, climatic zone, and

6 , ecological objectives, desired condition, and the economic potential and societal demand for forest products. Research will continue to produce improved models of crown fire initiation and fire behavior, along with quantified canopy fuels data, more complete understanding of departures from historical conditions, and further study of logistic, economic, and management constraints. Using the best available models, decision analysis tools, and experience to integrate basic landscape analysis with fuel-treatment prescriptions for specific Fire research can help determine the best treatment stands is currently the most effective approach to regimes for specific forest types, , soils, and other managing fuels and reducing crown-fire hazard. factors that influence forest health.

with forest type and with climate, soils, and other Summary factors that influence forest health. Ongoing research by Forest Service scientists Fuels Management Can Reduce Negative and their colleagues in other organizations Impacts of Fire, Benefit Forest Health, and continues to expand the knowledge base and Provide Social and Economic Benefits technology available to land managers charged with protecting America’s greatest natural legacy. Forest thinning and prescribed burning are two Fire science research, like medical research, allows land-management techniques long employed by practitioners to carry out treatments that are more and others to maintain forest health and specific and less invasive to the environment. In reduce wildfire risk. The benefits of these turn, field experience can inform the research so practices are supported by hundreds of scientific that the scientific base and real-world practice can investigations and years of professional field mature together. experience. These practices can be effectively combined in treatment regimes to improve forest health and reduce wildfire risk, while providing social and economic benefits. A science-based program of fuels management can reduce wildfire severity and protect environmental values, including soil, water, air, and wildlife habitat. For more information about this topic, call How long treatments will last and how often Susan Conard at 703-605-5255 or Mike Hilbruner treated areas need follow-up maintenance will vary at 703-605-5254.

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